Iron deficiency is one of the most common human nutritional disorders in the world today (see the website at who.int/nut/ida.htm; Yip, R. (1994) J. Nutr. 124: 1479S–1490S). Indeed, iron is an essential nutrient for virtually all organisms because it plays a critical role in important biochemical processes such as respiration and photosynthesis. Although abundant in nature, iron is often available in limited amounts because the oxidized form, Fe(III), is extremely insoluble at neutral or basic pH. This fact is of particular importance to agriculture because approximately one-third of the world's soils are classified as iron-deficient (Yi, Y. et al. (1994) Plant Physiol. 104: 815–820). Many “iron-efficient” plant varieties have iron uptake strategies (designated strategy I or strategy II) that are directed at solubilizing iron (Römheld, V. (1987) Physiol. Plant. 70: 231–234). Strategy II plants, which include all of the grasses, release Fe(III) compounds called “phytosiderophores” into the surrounding soil that bind iron and are then taken up into the roots. Most other iron-efficient plants use strategy I and respond to iron deprivation by inducing the activity of membrane-bound Fe(III) chelate reductases that reduce Fe(III) to the more soluble Fe(II) form. The Fe(II) product is then taken up into the roots by an Fe(II) specific transport system that is also induced by iron-limiting growth conditions. Furthermore, the roots or strategy I plants release more protons when iron-deficient, lowering the rhizosphere pH and thereby increasing the solubility of Fe(III). Thus, it would be desirable to take advantage of this understanding of iron-uptake strategies to produce plants which have increased iron-uptake capabilities.
In addition, metal ion pollution is perhaps one of the most difficult environmental problems facing the industrial world today. Unlike the organic and even halogenated organic pollutants, which can be degraded in the soil, metals are essentially nonmutable. The electrolytic, in situ immobilization and chemical leaching technologies for cleaning polluted sites are all very expensive, particularly in light of how vast some of these sites are. With the exception of approaches like vitrification, most in situ metal ion remediation schemes require some mechanism for increased mobilization of the metal ion. This raises the possibility of further endangering local wildlife or adjacent ecosystems not already affected. Thus, a need still exists for better methods for removing toxic pollutants from the soil.